A few comments to add to the helpful ones already showing up here.
Be prepared to become a hands-on fixit guy, or gal. Lots of little maintenance items can only be learned by doing it yourself, perhaps initially with help from an experienced older friend or relative. Don’t expect to rely on contractors except for tough things like bigger plumbing or electrical problems. When you have a vacancy, countless little items must be attended to, and you will need to get to know your property thoroughly in order to address them yourself. Tenants rightly expect near-perfection with everything working, and anything broken or unsafe to be fixed immediately. Vacancies can eat you alive, so minimize turnover. When you have a vacancy put your life on hold and get it rent-ready and show it to prosepcts ASAP.
All this suggests newer SFRs would be the place to invest. Very few duplexes, triplexes, and fourplexes have been built in recent decades, so where they exist, they have the maintenance problems that come with age. Plus, they tend to be in lesser neighborhoods.
